To maximize the transmission capacity of an optical fiber transmission system, a single optical fiber may be used to carry multiple optical signals in what is called a wavelength division multiplexed system (hereinafter a WDM system). The multiple optical signals may be multiplexed to form a multiplexed signal or WDM signal with each of the multiple signals being modulated on separate wavelengths referred to as channels. The channels are positioned relative to each other on a grid, e.g. as defined by the International Telecommunication Union (ITU), with an associated pre-defined channel spacing. Modern WDM systems have a high traffic capacity, for example, a capacity to carry 126 channels or more at 40 gigabits per second (hereinafter Gb/s) or more.
The optical fiber transmission system may include a relatively long trunk fiber segment that may be terminated at a transmitting and/or receiving trunk terminal. The optical fiber transmission system may further include one or more branching units situated along its trunk. Each branching unit (BU) may be connected to a branch fiber segment that terminates in a transmitting and/or receiving branch terminal. Each BU may include one or more optical add/drop multiplexers (OADM). Channels may be added to and/or dropped from the trunk fiber segment of the optical transmission system via the OADMs.
In WDM undersea optical networks, the deployment of OADM elements greatly increases the flexibility of network topology and traffic distribution. The ability to connect multiple stations via the same fiber pair (FP) by sharing and reusing bandwidth between different digital line segments (DLSs) is attractive to network operators due to its reduced system cost. Each DLS consists of a group of channels on an OADM FP that share the same transmit and receive terminals.
When information signals are transmitted over long distances, one or more amplifiers, e.g. erbium-doped fiber amplifiers (EDFAs), are provided to compensate for signal attenuation. The amplifiers used in some WDM systems (e.g., undersea systems) cannot easily be modified once installed and are initially configured to support a fully loaded system. In general, it may be desirable that the power per-channel be sufficient to provide an adequate signal-to-noise ratio in the presence of the amplified spontaneous emission (ASE) noise from the amplifiers, necessitating a high amplifier total output power for systems with high fully-loaded capacity. The amplifiers may thus be configured to provide an optical output signal at a nominal total optical power. As used herein, use of the term “nominal” or “nominally” when referring to an amount means a designated or theoretical amount that may vary from the actual amount.
The nominal amplifier output power level may be insensitive to the power at the input of the amplifier. As the amplifier input power varies over a wide range, the total amplifier output power may change very little around the nominal output power level. As additional channels are added, e.g. at a branching unit, the optical output power per channel may decrease. As channels are dropped, the optical output power per channel may increase.
Optical signals, while propagating through optical fibers, can experience nonlinear interaction. At sufficiently high values of optical power (e.g., more than 1 mW per channel), the optical signal may experience more distortion than at low optical powers (e.g., less than 1 mW per channel) which results in transmission penalty. Therefore, when channels are dropped, e.g., at a branching unit, the value of optical channel power may increase, and network communication performance may suffer. Partial channel loading of a chain of optical amplifiers may result in undesirable noise accumulation in parts of the transmission band and gain reshaping effects that also degrade channel performance.
If a cable fault occurs due to fiber cut or component failure, the fault interrupts traffic on all DLSs that pass through the location of such fault. Additionally, changes in channel power distribution will impact the performance of other DLSs on the same FP that are not directly affected by the cut. The severity of the impact depends on a number of factors, such as DLS length, wavelength allocation, properties of optical amplifiers and terminal equipment. Some channels on remaining DLSs may suffer performance penalties that will cause interruption of customer traffic. Unlike terrestrial networks, submarine systems may not be designed to have an alternative path to which traffic may be routed in case of a fault and thus may rely on a recovery mechanism to restore the performance of as many channels on the OADM FP as possible.